Economy of scale. Helium-3, like other gases, is a bulk product. The trade run into Frankfurt is also deemed by our logistics department as being most viable as a one-way trade run. It is simply less efficient for our transports to be making continuous runs to Manneheim.
A single, large movement of gas is more efficient for our organisation, and can be managed with a skeleton crew, plus escorts as required. This is particularly true given the shorter distance we have to cover between our operation, and Manneheim Station.
I hope that this satisfactorily answers your question.